Activation Development Features of Overburden Fractures and Stability of Water-resisting Layer in Close Coal Seams Mining

In the Jurassic coal field of Northern Shaanxi Province, close coal seams repeated mining can intensify activation development of overburden fractures, which easily leads to the instability of water-resisting layer and groundwater loss. In order to reveal the development characteristics of overburden fractures and the stability of water-resisting layer in close coal seams mining, the main coal-rock combination types based on water conservation mining of close coal seams in Jurassic coal field were proposed by using the method of measurement statistics, physical similarity simulation and theoretical analysis, and the fracture activation development law of repeated mining were analyzed. The water-resisting layer stability criterion of repeated mining was established. The research shows that according to the occurrence conditions and geological characteristics of the coal water in the two coal seams at the top of the coalfield, it can be divided into four types, thin and medium thick-thick type, thick - thin and medium thick type, thick - thick type, thin and medium - thick type. Based on the above four types of conditions, the failure characteristics of the water-resisting layer with upward and downward fracture development of mining overburden rock were obtained. The ratio of upward fracture development height to comprehensive mining thickness of repeated mining is generally 14-30, which is significantly reduced compared with the single coal seam mining, and the development depth of downward fracture is about 1.6 to 3.0 times of the comprehensive mining thickness. The stability criterion based on the height (depth) of the repeated mining fractures was established (When the thickness of water-resisting layer is greater than or equal to the sum of development height of fractures, development depth of fractures and safety layer thickness, the water-resisting layer is stable, and vice versa), and the water-resisting layer stability zoning of repeated mining in the Jurassic coal field in Northern Shaanxi was proposed, it includes three conditions: stable - stable, stable - unstable and unstable - unstable. The research results can provide a theoretical basis for the stability criterion and control of water-resisting layer in the Jurassic coal field in Northern Shaanxi.